that they may result from the combined action of the oxygen and nitrogen of the atmosphere on the organic materials in the process of burning. Fourcroy goes still further. In his work on "Chemical Philosophy," to which I have referred, he writes (translation):
"We do not understand the composition of potash. It has been suspected that it might result from a union of lime with nitrogen, because it is often found in vegetables mixed with this earth; but this theory, which I brought forward some fifteen years ago, has not been proved by any positive fact." It is interesting to go back and watch this groping in the dark for what is now positive knowledge, but the experience may teach us many a valuable lesson, and will at least help us to realize the intense enthusiasm with which, on October 6, 1807, Davy saw metallic globules running from a lump of caustic potash under the influence of the current of his new voltaic battery.
With this great achievement of Davy the formative period of the Lavoisierian system of chemistry may be said to have closed; but in this connection it is amusing to notice that in a chemical text-book studied in Harvard College by the class of 1815, and given me by the late Hon. John G. Palfrey, of that class, the alkalies and earths are included in the list of chemical elements, and Davy's discovery is only briefly referred to in a note.
Immediately after Davy's short but brilliant career, the science of chemistry took the form which it retained for nearly fifty years—a form in which it was first studied by all the older men of the present generation. The form was essentially that given by Lavoisier, and its chief merit was the simplicity of the classification, and the admirable nomenclature in which this classification was expressed. This nomenclature, which is to a great extent still retained, although the terms have lost most of their original significance, was devised by Lavoisier, with the co-operation of several of his associates, and adopted with the sanction of the French Academy of Sciences. It was a masterly production, and very greatly strengthened the hold which the system acquired at all the great centers of learning. The general features of the Lavoisierian system can be stated in few words.
Oxygen, which constitutes at least one half of the earth's crust, is the common cement by which all the elementary parts are held together. It is the universal acidifying principle, and the salifiable bases owe their peculiar relations to the same element as well. The elements may be divided into metals and non-metallic substances. The direct compounds of the non-metals with oxygen in different proportions are acids, while the compounds of the metals with oxygen are salifiable bases, and the compounds of the acids and bases are salts; and simple salts may still further combine with each other to form double salts. Thus, beginning with the